The present invention relates generally to measuring spectral energy in a cable transmission system and more specifically to measuring interference in such systems.
Cable transmission systems, such as cable TV networks, local area networks, telephone exchange systems, and the like, have traditionally provided separate services to customers. In the era of what has been called convergence, cable transmission systems are being adapted to provide video, data, and voice over a single system. For example, most existing cable TV systems provide one way analog video to customers in a forward path over a broadband distribution system. The system includes a headend that processes incoming video signals from satellites, land-line microwave transmissions, local station antennae feeds, and local studio signals and combines these signal into a broadband signal. The broadband signal is coupled into one or more trunk lines having branching feeder lines. The feeder lines contain taps for coupling the broadband signal from the feeder line to coaxial cables connected to set-top converter boxes or cable ready television sets on customer premises. One type of set-top converter box demodulates the RF signal to baseband for processing, such as decoding, and remodulates the signal for channels 3 or 4 of the television receiver. Another type of set-top down converts the incoming signal directly to channels 3 or 4. There are cable TV systems that provide basic two-way communications from the headend to the customer premises and back using a reverse path established through the set-top converter box. This reverse path is used for ordering pay-per-view events, movies, and the like as well as status monitoring of such system components as power supplies and amplifiers from the headend.
As more interactive services are developed for cable transmission systems, such as internet access, video-on-demand, telephony and the like, the cable transmission system providers are upgrading their systems for digital transmission of voice, data, and video. The telco industry is upgrading its systems for transmitting video over existing twisted-pair cable. Cable TV system operators are upgrading their systems to provide voice and data over existing coaxial cable systems or deploying hybrid optical fiber-coaxial cable (HFC) networks. An exemplary HFC system employs optical fiber as the backbone from the headend to hubs or remote headends in the system. The broadband signal at the headend is converted to an optical signal and transmitted over the optical fiber backbone. The hubs or remote headends convert the optical signal back to an electrical signal and can add additional signals to the broadband signal, reassign programming to new channels, amplify the broadband signal, and the like, and reconverts the broadband signal to an optical signal. Optical fiber trunk lines couple the signal from the hubs or remote headends to fiber nodes. The fiber nodes convert the optical signal into an electrical signal and re-transmit the signal over the coaxial cable lines to customer premises. Alternatively, fiber may be brought directly into the customer premises. The coaxial cable or optical fiber connection to the customer premise's may be connected to a splitter with one output being coupled to the set-top converter box or cable ready television and another connector being coupled to a cable modem. The cable modem may be connected to interactive devices, such as a single computer or internet device or connected to a local loop where multiple computers and/or internet devices are on the loop. Data from these devices are transmitted by the cable modem on the reverse path using a TDMA modulated carrier. A TDMA carrier is "bursty" in nature and is only present when transmission is occurring.
One problem the providers are dealing with in these new digital systems is RF noise. The RF noise of particular concern is interference entering either the forward or reverse path and is referred to as "ingress". This interference may be caused by any number of things, such a home appliances, citizen-band radio, paging services, public safety transmissions (i.e. police and fire communications), and the like. Regardless of the source of interference, the presence of ingress reduces channel operating margins. Each customer premise has the potential of becoming an interference source and/or an entry point for ingress, which could affect all reverse signals coming into the headend.
What is needed is a diagnostic method for measuring and monitoring the ingress in cable transmission systems, such as cable TV systems and telephone networks. Such a method should be able to acquire, analyze, and display acquired data representative of interference over specified frequency ranges that are selectable by a user. The method should provide a spectral trace of the acquired data as well as provide a frequency and amplitude information of the significant interference within the defined frequency range.